1. Technical Field
The present invention relates to a projection optical system for projecting light on a projection surface while zooming the light.
2. Related Art
In a projector, image light representing an original image displayed on a liquid crystal panel is projected on a projection screen via a projection optical system, thereby displaying the image. However, the projection image of the projector might have distortion such as keystone distortion caused depending on conditions (e.g., the position or the tilt angle of the projection screen with respect to the projection optical system, and the shape of the surface of the projection screen) of the projection screen. In the past, there have been proposed various kinds of technologies for reducing the distortion of the projection image by executing a correction process on the original image in accordance with the conditions of the projection screen (e.g., JP-A-2001-061121, JP-A-2001-320652, and JP-A-2004-140845).
Incidentally, the projection optical system used in the projector is generally provided with a zoom mechanism, and is capable of displaying the projection image while arbitrarily zooming the projection image. However, when controlling the zoom ratio by the zoom mechanism, the focal distance of the projection optical system varies. Therefore, in the case in which the focal distance of the projection optical system is used as an internal parameter of the projection optical system in the correction process of the original image described above, it is preferable that the focal distance of the projection optical system thus varied after the zoom ratio has been controlled is measured more accurately. Such a demand exists not only in projectors, but also in common in optical equipment provided with the projection optical system having zoom function. However, in actual situations, sufficient devices have not ever been made with respect to such a demand.